Lately, along with the discovery of Higgs particles and development of Big Bang and Inflation Theories, the construction project of the international linear collider (ILC), which is a linear accelerator with a length of as long as 30 to 50 km, has been in steady progress.
The core devices of the ILC are superconducting high frequency accelerator cavities, whose single unit is called a “9-cell cavity”. Each unit is composed of a center component 2 made of nine cells and end-group components 3 on both sides of a unit as shown in FIG. 1. The end-group component 3 is constituted by a HOM (High Order Mode) coupler 3c having a complicated shape and ports (a beam pipe 3a and a port pipe 3b) and so on for power input and its monitoring.
The HOM coupler 3c integrates, as shown in FIG. 2, a HOM cup 4 and a HOM antenna 5. That is, when a particle beam is accelerated in electro-magnetic fields and passes through the cavity, the HOM (High Order Mode) wave is excited and prevents the acceleration of the beam. This wave needs to be sucked out of the cavity and modulated. This function can be conducted by the HOM coupler (HOM moderator).
Primary materials of a 9-cell unit and the end-group component 3 are both pure niobium, one of rare metals. The main reason is that pure niobium has as high superconducting transition temperature as 9.2 K, and by using it at 2 K, there is a strong possibility to obtain a high acceleration voltage per unit length of a cavity, the most important superconductive cavity characteristics of ILC, due to easier acceleration of the particle beam.
Pure niobium is a material which is extremely expensive and tough for machining and press-forming The main reasons are a low plastic strain ratio in press-forming and seizure with tooling. The HOM antenna 5 is conventionally made into a final product by full machining or firstly into a near net shape semi-product by waterjet cutting then into a final product by cold forgoing.
The HOM cup 4 is produced by full machining or backward extrusion followed by machining and heat treatment or plural processes press-forming with final heat treatments.
All of them involve serious problems in terms of productivity and cost-effectiveness. Therefore conversion of production method to advanced whole press-forming has been strongly desired in order to sort out the issues.
Thus, the inventors have had R&D works concerning HOM cup 4 to attain the conversion of a production method to innovative ultra-deep drawing procedure, and have already filed domestic and international patent applications (Patent Documents 1 and 2).
However, the HOM antenna 5 is, as is judged from an appearance in FIG. 2D, a “tough-workable shape component” for press-forming procedure. Pure niobium, herein, is a “tough-workable material” both in mechanical cutting and press-forming Further the HOM antenna 5 is of a “plate” with an initial thickness of approximately 10 mm. These lead to high barriers to be sorted out.
In the HOM antenna 5, in order to have proper superconducting characteristics, dimensions are important, including plate thickness and R value (plastic strain ratio) at a variety of angles from the rolling direction of a plate material. In the conversion from machining to press-forming of the end-group components 3, R &D works of both “material technology” and the “plastic working technology” are simultaneously needed. The radius of perforation in a nearly square product is very small, stress concentration can easily be generated. Hence the occurrence of necking/crack, metal surplus/shortage, shape fixability and residual stress are expected to lead to severe forming difficulty.
Moreover, CP (chemical polishing) and EP (electrolytic polishing) are performed as a finishing process, wherein, for the purpose of reduction of load given, the surface condition without the presence of foreign objects and small amount of impurity elements on or slightly below the material surface should be properly arranged.
Thus, any working method of the HOM antenna 5 other than full machining or waterjet cutting followed by cold forging has been neither known nor established. Significant improvement of mass productivity and reduction of a manufacturing cost by means of conversion of forming procedure from machining or waterjet cutting followed by conventional cold forging is extremely required.
As a method for meeting the requirement, the present invention gives an achievement resulted from R&D works for the materialization of an idea which has not been tried. This is “innovative full press-forming” composing of advanced technologies of an “innovative shear-blanking method” and a subsequent “innovative forging method” for the conversion of prior methods to the full press-forming.
Firstly, the above R &D works excluded both conventional shear-blanking and fine blanking, because, in the former, a clearance is usually 5 to 10% of the plate thickness (t), thus, it was impossible to realize a required dimensional accuracy, while in the latter, due to elevated costs caused by an expensive exclusive machine and an expensive tooling die plus high technical difficulty, production efficiency comes to be a serious problem.
Prior to examination of the “innovative shear-blanking method”, the inventors evaluated a possibility of producing a near net shape semi-product using the “waterjet cutting” instead of mechanical cutting because it was seemingly available. Since relatively high speed and high efficiency are expected for the production of the near net shape semi-product by waterjet cutting, various examinations were conducted in parallel with the subsequent promising procedure by press-forming i.e. the well-known “cold forging”.
As a result, a couple of technical problems were recognized. The one was the presence of foreign objects on the metal surface found by SEM observation and EDX analysis after waterjet cutting followed by CP. They were also intruded into the matrix right below the surface (FIG. 3). It was clearly seen from a SEM image (FIG. 3A) that white point ranging in size from several μm to several tens of μm were scattered, and that a color tone of their periphery thereof was changed probably due to stress fields.
From the EDX measurement of an observed white spot encircled, for instance, in the SEM image (FIG. 3B), it was identified to be alumina, silica, iron oxide, magnesium oxide and the like. The existence of these foreign objects is speculated to be caused by “fillers” used in waterjet cutting to easily produce the near net shape semi-product. As long as this cutting method is used, remaining and intrusion of the fillers on and slightly below the surface of semi-products cannot be avoided.
When the fillers remain in the products, there is a serious concern that the occurrence of a high-frequency resonant mode is enhanced, which gives a unfavorable influence on the cavity performances and thus, there is no choice but to avoid the waterjet cutting to manufacture the near net shape semi-products. Moreover, it is undeniable that the waterjet cutting is poorer in productivity and cost effectiveness than the press shear-blanking In the case of HOM antenna 5, approximately 10 minutes are required to produce one piece, so that the waterjet cutting procedure is not suitable for mass production of several tens of thousands pieces of HOM antenna needed for ILC project.
Secondly, as the production method of a near net shape semi-product into a final product, availability of the conventional cold forging was investigated. However, as a result of experimental works, problems such as necking, dimensional irregularity, stress concentration and shape fixability (shear droop, bur, and metal surplus/shortage) were found in addition to a problem of seizure as well. Common factors of these problems are associated with “plastic metal flow of the formed material related to applied force” between the material and the tooling die.
Among them, local occurrence of the necking after the cold forging as shown in FIG. 4 is a serious problem in particular. Experiments were conducted by changing cold forging conditions regarding plastic formability. But the results showed the impossibility of complete avoidance of necking generation (exhibited by an ellipse in the FIG. 4).
Even if necking formation is of remarkably small probability, just a single necking deteriorates the function of the HOM antenna 5 to bring about serious damage to the whole operation of the accelerator. Therefore necking defect should be absolutely averted.
It is certain that the necking was directly caused by stress concentration, but it is not known yet which of insufficient strength of the material, poor ductility, deficient plastic metal flow, or a small margin of further deformation of the material is a primary factor.
Either remnants of fillers or necking generation is caused by the interaction between the material and its deformation. It is certain that each phenomenon deteriorates the control of the resonant frequency mode or superconductivity itself after combining HOM antenna with HOM cup and the following electron beam welding (EBW), so that remnant fillers and necking defects should be prevented. This is why R&D works developing an innovative production method of HOM antenna 5 by paying attention to both material and working/forming is extremely essential.